1. Field of the Invention
The present invention relates to a multi-wavelength locking method for a wavelength division multiplexing (WDM) optical communication network. In particular, the present invention relates so a multi-wavelength locking method and apparatus for a WDM optical communication system that can lock wavelengths of optical signals by producing pilot tones by applying a sine-wave current to a plurality of transmission lasers having different wavelengths, passing the optical signal through a Fabry-Perot etalon filter, and then Fourier-transforming the filtered optical signal.
2. Description of the Related Art
A wavelength division multiplexing (WDM) optical communication system is an effective super-high-speed and super-wide-band transmission system that multiplexes optical signals of various wavelengths and transmits the optical signals through one optical fiber.
Currently, due to the rush of various kinds of data traffics including Internet, the WDM optical communication system has a reduced channel spacing, and accommodates lots of channels.
Thus, it becomes important to accurately control frequencies of respective lasers in the system. For example, if the channel spacing is 50 GHz, the frequencies of the respective lasers operating at a transmission speed of 10 Gb/s should be locked within 2.5 GHz.
However, frequencies of semiconductor distributed feedback (DFB) lasers widely used may vary in the range of tens of GHz due to their aging even though the current, temperature, etc., for driving the lasers are kept constant, and thus the frequencies of he respective lasers should be locked to reference frequencies.
Recently, there have been proposed methods of improving the frequency Locking state of the WDM lasers using a fiber grating, arrayed waveguide grating (AWG), etalon filter etc., as a frequency reference element.
However, to lock the respective WDM lasers to the optical frequencies recommended by International Telecommunication Union (ITU), fiber gratings whose number is as many as the number of channels being used are required, and the reflection and transmission characteristics of the respective fiber gratings should coincide with the standard frequencies.
According to the wavelength locking method using the AWG, the WDM lasers can be simultaneously locked, but the maximum number of channels that can be locked is limited by the free spectral range (FSR) of the commercialized AWG. Also, since the respective band-pass characteristics of the AWG are slightly different from one another, the wavelength locking state of the respective WDM channel may deteriorate.
Meanwhile, the etalon filter synchronized with the reference frequency can provide absolute frequencies consistent with the standard frequencies without using the frequency-locked lasers.
By adjusting the incident angle, the etalon filter that coincides with the reference frequency and thus provides certain frequencies with the equal spacing can be easily provided.
An automatic wavelength locking system using the synchronized etalon filter is disclosed in Korean Patent Application No 97-17558 “Cold-start type wavelength division multiplexing optical transmission system and standard frequency generating method thereof” . However, in the same manner as the most locking methods, it has problems in that it requires lock-in amplifiers as much as the number of WDM channels.
As the number of WDM channels, the system becomes complicated and non-economical.
If the lock-in amplifiers are used for locking the wavelengths, frequency offsets may exist due to the transmission function of the etalon filter.
This transmission function of the etalon filter is caused by an intensity modulation of the laser due to the pilot tone. In order to suppress the intensity modulation effect of the laser due to the pilot tone, the phase of the standard signal inputted to the lock-in amplifier should be accurately controlled to be 0.5 π.